Our prior studies of carbocyclic analogs of purine nucleosides and of a few pyrimidine nucleosides have uncovered significant anticancer and antiviral activity among compounds with this structure, in which the furanose ring of nucleosides is replaced by a cyclopentane ring. The observed activities by certain carbocyclic analogs, the importance of pyrimidine nucleosides among clinically active anticancer drugs, and the preeminence of nucleosides among clinically useful antiviral agents indicate a potentially important role for carbocyclic pyrimidine nuleoside analogs. For these reasons, this proposed, collaborative research program includes the synthesis of new carbocyclic pyrimidine nucleoside analogs that are either congeners of the active carbocyclic analogs or are analogs of known, active pyrimidine nucleosides. The new analogs are being evaluated in vitro for anticancer activity and for antiviral activity. Analogs that show significant activity in vitro are to be further evaluated in vivo. Biochemical studies are designed to determine if active analogs selectively inhibit virus-induced enzymes.